to see if an Ethernet link is throwing more than a handful of initial errors, you can use Terminal command:
my main connect uses en5 (as shown in system preferences > network:
netstat -I en5
This is the resulting output. Counters are In-packets, In-errors, Out-packets, out-Errors. There should never be more than handful of errors from starting up, and in most cases, NONE.
Name Mtu Network Address Ipkts Ierrs Opkts Oerrs Coll
en5 8163 <Link#4> 00:01:d2:1a:00:dd 696697 0 484301 0 0
en5 8163 grantsmacpr fe80:4::461:ea0d: 696697 - 484301 - -
en5 8163 192.168.0/23 192.168.0.204 696697 - 484301 - -
If the link were running beyond its ability to run stably, for example it auto-speeded to 10GB but the cabling could only reliably support 2.5GB, we woud see non-zero errors counts, and errors increasing over time. (and possibly, disconnecting)